1. The Field of the Invention
The present invention relates to a method of making low-strain or strain-poor single crystals with a hexagonal structure, especially corundum single crystals, by the Czochralski method and a subsequent tempering process, to the low-stress single crystals themselves obtained by the method, and to their use for making electronic and electrical components.
2. Description of the Related Art
The manufacture of III-nitride semiconductor elements has made the development of many electronic components for high temperature engineering and high-energy engineering, such as laser applications, possible. Above all blue and white light emitting diodes (LED) that produce high light intensity were made possible by these elements, also their mass production. The principal problem with this is sufficient availability of suitable substrate material. A suitable substrate must, above all, have a high transparency and a sufficient resistance to corrosive action during manufacture of semiconductor structural elements. It must have sufficient form-stability at temperatures above 1300 K, in order to permit a uniform semiconductor layer growth.
Thermal properties, such as thermal expansion and heat conduction, must similarly have suitable values. Moreover a so-called mismatch between the crystal lattice of the substrate and the crystal lattice of the crystalline gallium nitride layer must be as small as possible, so that the gallium nitride layer can be epitaxially applied to the substrate. Currently sapphire is used as the substrate. The physical properties of sapphire are close to those of GaN and other semi-conductor variants used with it, such as AIN, GaN, InGaN or InGaAl. So-called wafers are used as substrates. They are thin substrate disks with diameters of a few inches (2 to 4″). The smallest mismatch between sapphire and GaN results, when wafers with a <0001> orientation are used as substrates. The mismatch of the lattice constants of the sapphire in relation to the GaN is minimal with this <0001> orientation. In this <0001> orientation the [0001] surface is the wafer surface and the crystallographic c-axis is perpendicular to the wafer surface.
The so-called Czochralski method, in which a seed crystal is immersed in melted raw material and slowly drawn from this melted raw material, is a suitable method in wide spread use for sapphire growth. This Czochralski method has the advantage that a true crystal growth and thus an ordered structure is possible in contrast to other known crystal growth methods. It is generally problematical that a single crystal of high quality can only be made by the Czochralski method when the seed crystal is withdrawn from the melted raw material parallel to the crystallographic m-direction of the sapphire. This m-direction is oriented at an angle of 90 degrees with respect to the c-axis. A crystal growth also partially occurs according to the Czochralski method, when the drawing direction is oriented at an angle of 60° to the c-axis (r-direction). In this case it is disadvantageous that the desired wafer in the <0001> orientation must be costly obliquely cut out from the thus grown single crystal. This reduces the yield and requires great processing effort in sapphire because of its great hardness. Moreover the wafer so obtained has unsymmetrical relaxation behavior of the intrinsic stress and thus an unsymmetrical deformation of the wafer during subsequent temperature treatment. This behavior reveals itself as interference with later epitaxial growth process, for example growth of GaN on the wafer. It is due to the high temperatures occurring during epitaxial growth processes and thus causes the wafer to deviate from its required planarity. In the Czochralski method in which the crystal is drawn in the m-direction a comparatively great material waste measured in broken crystal volume in comparison to the obtained wafer surface area results. Also the wafer quality is disadvantageously influenced by deformation as a result of stress relaxation at temperatures above 1270 K. The deformation by stress relaxation reveals itself already during the grinding and polishing processes during manufacture of the wafer.
Up to now good results regarding crystal quality could not be obtained with the Czochralski method in the c-axis direction because the growing speed must be minimized according to the axial temperature gradient at the phase boundary. Also no economical manufacture of sapphire crystals was possible according to this process generally on account of the limited growing speed. The poor material quality was largely due to the fact that the growth occurred at the atomic smooth [0001] surface. However flaws arise already on this surface with only very little surface energy in this crystal system. Small angle grain boundaries also arise making the substrate material unusable for high quality electronic components.
The manufacture of as-uniform-as-possible stress-free oxidic single crystals is described in DD-A 202 901. A very flat temperature gradient is set in the entire growth chamber in a high frequency heating system. However it has been shown that this method is not suitable for mass produced products, such as substrates for semiconductor elements.